The present invention relates to an optical device for proximity coupling between two waveguides integrated in a substrate and, more particularly, to such a device applicable to the realization of multiplexers/demultiplexers or filters of reduced dimension. The present invention relates also to an integrated optical component comprising a combination of such multiplexers/demultiplexers and filters.
Proximity couplers such as that which is represented schematically in FIG. 1A in the drawings attached to the present patent application are known. Such a coupler is currently being utilized in the multiplexing/demultiplexing of two signals of different wavelengths. It comprises two waveguides 1,2 integrated by exchange of ions such as the ion thallium, for example, in a glass substrate 3 or in a crystalline substrate. The ion exchange is brought about by means of a mask which defines the form of the waveguides. As is shown, these latter consist of straight interaction segments 1.sub.1, 2.sub.1 parallel and neighboring, and approach segments 1.sub.2, 1.sub.3, and 2.sub.2, 2.sub.3, respectively, connected to inputs/outputs or "ports" 4, 5, 6, ports 5 and 6 being separated by a distance fixed by the diameter of coated optical fibers (not shown) which are attached to these ports by a technique of "pigtailing", for example.
Thus, signals of wavelengths .lambda..sub.1, and .lambda..sub.2 (.lambda..sub.1 &lt;.lambda..sub.2) entering by port 4 exit separately through ports 5 and 6, the signal at one of the wavelengths passing into waveguide 2 because of the crossing over of evanescent waves in the substrate between the straight interaction segments 1.sub.2 and 2.sub.2 of the guides, in particular. The response curve of such a coupler typically takes the form represented in FIG. 2 which makes evident two attenuation peaks centered along the wavelengths .lambda..sub.1 =1300 nm and .lambda..sub.2 =1550 nm, as is the case, for example, when one utilizes such a coupler in optical fiber telecommunications. It is clear that the signal at wavelength .lambda..sub.2 =1550 nm is very attenuated at port 6, when the signal at wavelength .lambda..sub.1 =1300 nm in practice is not, and inversely at port 5. One can thus separate, for example, a signal carrying an audio signal from a signal carrying a video signal, transmitted together in the same optical fiber. The same signals can be combined in the same fiber advancing in the opposite direction in the coupler of FIG. 1A.
As depicted in this FIG., the approach segments of the proximity coupler are shaped in such a manner as to absorb the spatial displacement of the inputs/outputs 5, 6 of the coupler, currently on the order of 350 .mu.m for reducing the separation s of the straight segments of guide to the order of ten microns, typically 8-20 .mu.m, thereby reducing to a minimum the curvature losses, which requires the use of a large radius of curvature R for these approach segments, typically on the order of 100 mm. Two successive sections of the guides exhibiting this curvature and connected by an inflection point are therefore normally necessary in order to form an approach segment. Because of the large radius of curvature which is used, these approach segments therefore have a significant longitudinal dimension, representing typically 3/4 of the total length of the coupler, which can then easily reach approximately 22 mm when the length 1 of the straight interaction segments (1.sub.1, 2.sub.1) Of the waveguides, which assures the essentials of coupling, is on the order of 5 to 10 mm, approximately.
Concerning integrated devices, made with the aid of masks, there exists a need for a more compact configuration of these devices, allowing the arrangement of a greater number on the same planar area of a glass substrate, for example, which permits a proportional increase in the volume of production for these devices.
More compact devices would permit the manufacture of integrated optical components combining multiplexers/demultiplexers, filters, etc., which themselves benefit by a reduction in dimension.
The present invention therefore has as its object the furnishing of an optical device for proximity coupling, especially permitting the realization of directional proximity couplers or multiplexers/demultiplexers in integrated optics with reduced dimensions.
The present invention also has as its object the furnishing of such a device which should be equally applicable to the realization of integrated optical filters with reduced dimensions, and to the realization of integrated optical components combining, for example in parallel or in series, such multiplexers/demultiplexers and of such filters.
We achieve these objects of the invention, as well as others which will appear upon reading the description which will follow.
According to one aspect of the invention, we provide an integrated optical device for proximity coupling between two waveguides in order to separate or combine signals of different wavelengths, including a coupling region where most of said coupling takes place comprising two straight interaction segments of said waveguides parallel and near to one another, and further comprising one or more approach segments of said waveguides coupled to said coupling region on each of its two sides by connection to one or more ends of said straight interaction segments, characterized in that said approach segments on one side of said coupling region are free of any inflection point, and the coupling length (1) and separation (s) of the straight interaction segments are adjusted in order to compensate for the variation of the coupling between the guides along said approach segments resulting from the removal of the inflection points.
The removal of the inflection points is a consequence of the reduction of the length of the curved sections of the approach segments which generates a substantial benefit in the longitudinal dimension of the device, this latter without modification of the coupling of the guides, thanks to a compensating modification of the coupling of the straight interaction segments, as will be seen later.
According to another aspect of the invention, we provide an integrated optical device for proximity coupling between two waveguides in order to separate or combine signals of different wavelengths, including a coupling region where most of said coupling takes place comprising two straight interaction segments of said waveguides parallel and near to one another, and further comprising one or more approach segments of said waveguides coupled to said coupling region on each of its two sides by connection to one or more ends of said straight interaction segments, characterized in that said device comprises a filter, and at least one of said approach segments comprises an exit end segment which is curved without any inflection point and terminates in the substrate of said integrated optical device, the length and curvature of said exit end segment being fixed to avoid recoupling of the light energy which is transmitted through said segment into said substrate.
According to still another aspect of the invention, we provide an integrated optical device for proximity coupling between two waveguides in order to separate or combine signals of different wavelengths, including a coupling region where most of said coupling takes place comprising two straight interaction segments of said waveguides parallel and near to one another, and further comprising one or more approach segments of said waveguides coupled to said coupling region on each of its two sides by connection to one or more ends of said straight interaction segments, characterized in that
the approach segments at least on one side of said coupling region are free of any inflection point,
at least one of said approach segments without inflection point is rectilinear and collinear with the straight interaction segment to which it is connected and is connected through its opposite end to an input/output of said device, and
at least one of said approach segments comprises an exit end segment which is curved without any inflection point and terminates in the substrate of said integrated optical device, the length and curvature of said exit end segment being fixed to avoid any recoupling of the light energy which is transmitted through said segment into said substrate.
In accordance with the process for making such devices operating at predetermined wavelengths, the location of such wavelengths is adjusted by a heat treatment procedure after the waveguides are integrated in the substrate of the device.
It is an important characteristic of the device according to the invention that the coupling length and the separation of the straight interaction segments of the guides are adjusted to compensate for the variation in coupling of the approach segments with respect to the coupling of approach parts having inflection points, due to the omission of these inflection points.
The device, in accordance with the invention, is applicable to the realization of a proximity coupler or multiplexer/demultiplexer of two signals comprising on the one hand an input/output attached at one end of one of the two waveguides and, on the other hand, two inputs/outputs attached each to one of the other ends of the two waveguides, the coupling being characterized in that, on the side of the apparatus which comprises a single input/output, the approach segments are devoid of inflection points.
The device in accordance with the invention is also applicable to the realization of a filter for extracting one of the input signals and for transmitting the other, this filter being characterized in that an approach segment, rectilinear and collinear with a straight interaction segment on one of the waveguides, is attached to an input of the filter.